Disc, drum, and conical head chippers are basic equipment in sawmills, used for processing logs into useable lumber. They employ revolving chipper heads in either disc, drum, or conical configurations to which, typically, a number of knives are attached. In a disc chipper (or chipper disc) the knives may be attached to the periphery of the disc or the sides of the disc. In drum and conical head chippers, the knives are attached to the periphery of the drum. Any of these chippers will be referred to hereinafter simply as a “chipper,” and either a disc, drum, or conical head will be referred to simply as a “cutting head” or “head.”
The knives are typically clamped to the head between a pair of clamping members, an “outer” or “upper” clamping member, and an “inner” or “lower” clamping member that is attached to the base of the chipper. The knife has front and back sides, the former facing the direction of rotation of the head. The upper clamping member contacts the back side and the lower clamping member contacts the front side, clamping the knife therebetween.
The wood article to be chipped by the chipper is fed into the chipper at a given feed speed, the cutting head revolves at a given rotational speed, and the knives have a given knife density, i.e., they are angularly spaced apart from one another on the head a given amount. These three parameters determine the size, particularly the length, of the chips. While the wood article is chipped fundamentally to shape it into useable lumber, the chips themselves have economic value as components of engineered or manufactured wood products, such as pulp and paper, fiberboard, and oriented strand board.
The knife is typically removed from the chipper head by removing the upper clamping member. In some more advanced prior art systems, the knife can be removed by a threaded adjustment that pivots the lower clamping member, such as described in Swartwood et al., U.S. Pat. No. 5,979,522.
FIG. 1 shows, in cross-section, a generic knife assembly 2 for clamping a chipper knife. The knife assembly 2 is mounted to a chipper head 4. The assembly has four main components, a knife 5, an upper clamping member (or “clamp”) 6, a lower clamping member (or “holder”) 7, and a replaceable wear insert 8. The clamp 6 and holder 7 clamp the knife, and a bolt 11 passes through the clamp and into, or through, the holder.
The assembly rotates in the direction “R.” The knife has a front side 3 and a back side 4, the front side facing the direction of rotation R. Chips cut by the knife follow the path “P” and contact a wear surface 9 of the wear insert 8. The wear surface becomes worn as a result of this contact. The wear surface, since it is closest to the knife, experiences more wear than the holder 7.
The wear insert 8 is substantially smaller (less massive) than the holder 7. For this reason, and because the greatest amount of wear occurs in the localized area of the wear insert, it is economically advantageous to provide, in addition to the holder 7, the wear insert as a disposable part. The wear insert is also, typically, substantially smaller than the clamp. The wear insert may or not be bolted to the holder; however, it is not bolted to the clamp.
The wear insert is typically provided with a “wear coating,” that is applied to and hardens the wear surface 9. One example is referred to more particularly as a “hardsurface,” which results from hardsurfacing, i.e., the application to the wear surface 9 of a diamond carbide powder coating. With a wear coating, the wear surface is harder or more wear resistant than the tool steel of which it, and particularly the holder, is formed. Alternatively, the entire wear insert may be formed of a harder or more wear resistant material.
The term “counterknife” is sometimes used to refer to a wear insert. However, a counterknife is defined more generally as a part used for breaking and deflecting chips cut by the knife, and the term has consequently been applied to holders in assemblies that do not contain a separate wear insert. Herein, the term wear insert is used to refer to a specific, relatively small component of an assembly like the assembly 2.
FIG. 2 shows two important parameters relating the knife 5 and wear insert 8. The impact angle θ determines the “impact” of the cutting head on the chips. In a “high impact” configuration, θ=90 degrees and the wear surface 9 is a blunt obstacle to chip flow. To provide lower impact configurations, θ is increased resulting in a more gentle path for chip flow. It is desired to break the chips with only as much force as is necessary, to minimize wear and damage to the chips, so the impact angle is adjusted for various factors, including the strength of the wood being chipped.
The other important parameter shown in FIG. 2 is the depth “D” of the wear insert relative to the cutting edge CE of the knife. This parameter is adjusted to be appropriate for the chip length mentioned above.
Referring back to FIG. 1, the wear insert 8 is indexed to the holder 7 by a side of the wear insert 10a abutting the holder at a seat 10b of the holder. As the insert wears and the wear surface 9 erodes, the surface retreats in the direction of the seat 10b and the depth D is undesirably increased. When the wear has become unacceptable, the wear insert is removed and either the wear surface renewed or the part replaced.
It may be noted that the above discussion is based on a simplifying assumption that the knife does not wear appreciably relative to the wear insert. This is generally not true, so the actual relationship between wear of the wear insert and the depth parameter “D” is of course more complex than indicated.
Providing the wear insert, especially as a disposable part in a knife assembly like the assembly 2, is economical, however, it remains that the part must be replaced or reworked at intervals, and it would be desirable to increase the service life of the wear insert.